There is an interesting vulnerability in how Firefox handles bookmarks.
The flaw allows the attacker to steal credentials from commonly used
browser start sites (for Firefox, Google is the seldom changed default;
that means exposure of GMail authentication cookies, etc).

The problem: it is relatively easy to trick a casual user into bookmarking
a window that does not point to any physical location, but rather, is an
inline data: URL scheme. When such a link is later retrieved, Javascript
code placed therein will execute in the context of a currently visited
webpage. The destination page can then continue to load without the user
noticing.

The impact of such a vulnerability isn't devastating, but as mentioned
earlier, any attention-grabbing webpage can exploit this to silently
launch attacks against Google, MSN, AOL credentials, etc. In an unlikely
case the victim is browsing local files or special URLs before following a
poisoned bookmark, system compromise is possible.

Thanks to Piotr Szeptynski for bringing up the subject of bookmarks and
inspiring me to dig into this.

Self-explanatory demo page:
http://lcamtuf.coredump.cx/ffbook/

This is being tracked as:
https://bugzilla.mozilla.org/show_bug.cgi?id=371179